1. Field of the Invention
The present invention generally relates to the field of valve devices. More specifically, the present invention relates to a valve device for electrically regulating a pressure in a hydraulic fluid flow through the valve device in response to an actuating force during a normal operation and mechanically regulating said pressure during a fail-safe operation when no actuating force is present. In particular, the valve device is used in pilot valve assemblies in electrically controlled hydraulic damper devices.
2. Description of the Related Art
Electrically controlled hydraulic damper devices or shock absorbers may comprise pilot valves electrically actuated by, for example, a solenoid. The pilot valve is used to control a main valve, which in turn adjusts a flow of hydraulic fluid or damping medium between the working chambers, i.e. the rebound and compression chambers, of the shock absorber and thereby the damping characteristics. Such a shock absorber should preferably be able to handle electrical or mechanical malfunction of the solenoid or the control system without leaving the valves in an open state. Leaving the valves in an open state may result in a substantially unrestricted flow of hydraulic fluid between the damping chambers and consequently a lack of damping force.
A shock-absorber involving a fail-safe operation or mode in order to handle above mentioned malfunctions is presented in EP0400395 where a valve arrangement with two valve seats is suggested. In a normal operation or mode, a valve body is actuated by the solenoid in order to variably adjust a first valve restriction area when a first portion of the valve body interacts with a first valve seat. In a fail-safe operation mode or fail-safe mode, i.e. during fail-safe operation, a second portion of the same valve body is forced by spring means against a second valve seat thereby blocking the flow between the second portion of the valve body and the second valve seat. The valve arrangement also includes a through hole or passage with a fixed or constant restriction area to allow a small flow there through during fail-safe operation or the fail-safe operation mode. However, during the fail-safe operation mode, a high pressure difference over the through hole or passage can easily be generated due to the fixed restriction area. In other words, in the case of an increase in the amount of hydraulic fluid that needs to be displaced between the compression and rebound chamber, the fixed restriction area is unable to compensate for the change in the flow volume or amount of the flow. Thus, in addition to force exerting spring means, the pressure difference forces the second portion of the valve body against the second valve seat even further thereby generating an increase in the force that the second portion of the valve body exerts against the second valve seat. Thereafter, when the solenoid becomes active again the valve arrangement returns to its normal mode or operation. However, due to the increased force exerted on the valve body, the force required to transition from fail-safe operation or mode back to normal operation or mode during a pressurized state, i.e. during a driving state, may be difficult to generate by the solenoid. The valve arrangement is also likely to cause an abrupt transition with undesired pressure shocks in the system.
One attempt to solve the problem of transitioning between the two modes or operation modes is presented in EP0708268 where a disc-shaped member is used to block the radially arranged pilot passages in fail-safe operation mode. Due to through holes or passages in the disc-shaped member, the pressure acting on both sides of the member is essentially the same, and thereby the transition from fail-safe operation mode to normal operation mode does not require overcoming a force generated by a pressure difference as in EP0400395. However, one problem with the approach in EP0708268 is that relatively large plays or scopes (a gap between the disc-shaped member and the sliding surface of valve chamber walls and/or clearance in the interface between the plunger and the valve body) may be required in order to achieve smooth sliding operation. Such large plays or scopes are negative with respect to leakage in the interface between the disc-shaped member and the radial pilot passage(s). In particular, the large plays or scopes pose problems with the embodiment where an inclined surface is used to receive the disc-shaped member in fail-safe operation or mode. In this embodiment, there is a risk that the disc-shaped member is received in an inclined position, resulting in unpredictable closing of the pilot passage(s) and difficulties when returning to normal operation or mode.
Another solution is presented in EP1759256 which discloses a valve arrangement with two valve bodies arranged to adjust two cross sections arranged in series and which are opened and closed in a reciprocally proportional fashion. Similar to the problem above, there is a risk in this solution that the disc-shaped valve body may be positioned in an inclined position relative the other valve body resulting in an unwanted and unpredictable valve function.